So why do you think the Ryder kids become Pathfinder?

Part of me thinks Cora will have to take on some unexpected other duties. Maybe the fact that Sloane has defected makes an impact in some way that forces her into a role that maybe Sloane would have filled.

I'm guessing dad goes MIA. Cora gets knocked unconscious and while she's out the Initative promotes Ryder temporarily. He/she does a good enough job to be given the role full time.

thats stupid when she is part of the team and will be a squadmate 

The older protagonist didn't seem to be a problem in The Last of Us. But yeah Joel was his own character and games are still afraid to let a choice based game get an older protagonist.

Yeah, Joel was unusual in that sense, being late 40s early 50s. It was a father/daughter sort of story -- if it was just "Joel and the Apocalypse" I'm pretty sure he'd be a savvy 20-some-year-old.

Shepard was quite unusual in being a hero protagonist at 30 or so.

Cora is a squadmate in Peebee's recruitment, so I don't think she was hold on to something or unconscious.
Beside, I think they stated on twitter that it might have something to do with our family, but I'm not sure. I think it'll be that or something similae with SAM, expecially because even without Cora there's still Liam, so there should be a reason why they picked Ryder regardless.

Concise, direct, plausible. I like it.

It's a family run business, obviously. V. quaint.

That genetic link thing makes no sense at all. Why make Harper your second in command at all then if they can't have full access to SAM? Surely this would have all been arranged prior to launch. 

I would think due to a rebellion on the nexus (which is likely given the comic book) that Alex Ryder is no longer able to fulfill his duties as pathfinder. As one of the main leaders of the expedition, he has to be reassigned to the nexus, possibly on the council or something equally important. 
I mean, also why else would the head of nexus security, sloane kelly be running some random 'humble' outpost in the helios cluster unless she was involved in a rebellion against the council. 
I think the same reason Sloane Kelly is no longer nexus security is the reason Alec Ryder is no longer the pathfinder.

One of the reasons for the rebellion might be that one the the ARKs (or more) that was required to power the nexus upon entry into andromeda has gone missing and people are panicking, or believed it was sabotage (remember pre-ME3, the races didn't all trust one another). Or the council i.e. Jin, made a wrong decision about something in andromeda and the council was split leading to a rebellion on the nexus. 
Therefore, Alex Ryder appoints his son/daughter as pathfinder under the guidance of Harper. 
This makes far more sense than everyone elses theories. 
Since Harper was present when they discover the first vault at the beginning of the game, she probably wasn't MIA or uncouncious as people suggest .

Maybe this links in to the other theory flying about
Maybe someone high up in the Initiative finds out and decides she can't be trusted? Maybe it's also why Alec goes MIA.

Or maybe not....

Does the name "Geralt of Rivia" tell you anything? I hear he is quite popular
But in all seriousness, I don't dislike playing an older character, however if I have a choice I prefer either young protagonist or one with ambiguous age.

on topic: why do I think we become pathfinder? I hope we have to prove ourselves, being special is starting to be tiresome

Cora is a squadmate and a romance option for Scott, so no she will not be going missing. Alec on the other hand I am guessing he goes missing, if he does go missing though I don't know if we will end up finding him again later on